Abstract:

Post mid-Cretaceous sedimantary basins developed on the northern margin of the South China Sea display unique characteristics in their spatial and temporal distribution. Three basin evolutionary cycles which started in Late Cretaceous, Late Eocene and Middle Miocene respectively, have been recognized.Each of the cycles resulted from a seperate rifting episode. Interestingly,orientations of the major trends of the basins varied from predomenantly NE for the first cycle through NE-EW for the second to NE-EW for the third. Consequently, the early configuration of an individual basin has been apparently superimposed by the later events, resulting in a very complex pattern. Mechanical analysis suggests that the NE trending basins of the first cycle were probably the result of rifting along the extensional direction in a NW tensional terrane, whereas the NE-EW and EW-NW couples of the second and third cycles can be best explained by extension along Riedel shear and tensional components under a sinistral divergent wrench regime whose azimuth has rotated clockwise through time. Therefore, major changes in regional stress field are indicated. The tectonic setting related to the first basin cycle is inferred to be a intraarc ard back-arc extensional domain induced by dip-steepenning of subducted oceanic slab when the convergence rate between Pacific and Eurasian Plates decreased in Late Cretaceous. The collision of Indian Plate with Eurasian Plate about 40 m.y. ago and the following penetration of the former into the laterled to the large left lateral motion along Red River Fault and the clockwise rotation of Indochina as Indochina block and South China block were extruded successively by the collision, thus initiating the second and third basin evolutionary cycles.